Why is this character's arm moving when I edit its armatutre?
If merely entering then leaving edit mode on an armature leaves lasting changes to your pose, then you probably have a dependency loop-- some place where bone A depends on bone B which depends on bone C which depends on bone A (for example).  That's not going to work.
To rule in/out dependency loops, open a console window (from main menu's &quot;window&quot; menu) and look for any &quot;dependency cycle&quot; warnings.  If you have well named bones, you should be able to see where the loop exists.
Changing parenting on bones could easily create a dependency loop-- it interacts with constraints.  But changing parenting on bones also changes how they interpret their animation data.  You didn't ask about this, but if you change your parenting, don't expect your animation to be the same.